1. Field of the Invention
The present invention relates to a method of manufacturing a microlens. Moreover, the invention relates to a method of manufacturing a solid-state image pick-up unit comprising the microlens.
2. Description of the Related Art
FIG. 7A is a block diagram showing a main part of a solid-state image pick-up device incorporating a solid-state image pick-up unit and FIG. 7B is a schematic plan view showing a structure of the solid-state image pick-up unit. Moreover, FIG. 7C is a schematic sectional view showing a part of a pixel array portion of the solid-state image pick-up unit comprising a microlens.
Reference will be made to FIG. 7A. The solid-state image pick-up device comprises a solid-state image pick-up unit 51 for generating a signal charge corresponding to an amount of a light incident for each pixel and supplying an image signal based on the signal charge thus generated, a driving signal generating device 52 for generating a driving signal (a transfer voltage) to drive the solid-state image pick-up unit 51 and supplying the driving signal to the solid-state image pick-up unit 51, an output signal processing device 53 for carrying out a processing such as a reduction in a noise, a white balance or a data compression over the image signal supplied from the solid-state image pick-up unit 51, a storage device 54 such as a storage card connected to the output signal processing device 53 and serving to store the image signal, a display device 55 such as a liquid crystal display device for displaying the image signal, a transmitting device 56 to be an interface for transmitting the image signal to an outside, and a television 57 for displaying the image signal if necessary.
The solid-state image pick-up unit roughly includes a CCD type and an MOS type. The CCD type transfers an electric charge generated in a pixel through a CCD. The MOS type amplifies and outputs the electric charge generated in the pixel by an MOS transistor. Description will be given by taking the CCD type as an example, which is not particularly restricted.
A signal supplied from the driving signal generating device 52 to the solid-state image pick-up unit 51 includes a horizontal CCD driving signal, a vertical CCD driving signal, an output amplifier driving signal and a substrate bias signal.
Reference will be made to FIG. 7B. For example, the solid-state image pick-up unit is constituted to include a plurality of photosensitive portions 62 disposed in a matrix, a plurality of vertical CCD portions 64, a horizontal CCD portion 66 coupled electrically to the vertical CCD portions 64, and an amplifying circuit portion 67 provided on an end of the horizontal CCD portion 66 and serving to amplify an output charge signal from the horizontal CCD portion 66. A pixel array portion 61 is constituted to include the photosensitive portion 62 and the vertical CCD portion 64.
The photosensitive portion 62 is constituted to include a photoelectric converting unit (photodiode) and a reading gate. The photoelectric converting unit generates and stores a signal charge corresponding to an amount of an incident light. The signal charge thus stored is read from the reading gate onto the vertical CCD portion 64 and is transferred in the vertical CCD portion 64 (the vertical transfer channel) toward the horizontal CCD portion 66 (in a vertical direction). The signal charge transferred to a terminal of the vertical CCD portion 64 is transferred in the horizontal CCD portion 66 (a horizontal transfer channel) in a horizontal direction, and is amplified in the amplifying circuit portion 67 and is taken out.
Reference will be made to FIG. 7C. For example, a p-type well layer 82 formed on a semiconductor substrate 81 to be an n-type silicon substrate is provided with a photoelectric converting unit 71 constituted by an n-type impurity addition region and a vertical transfer channel 73 to be an n-type region which is adjacent thereto through a p-type reading gate 72. A vertical transfer electrode 75 is formed above the vertical transfer channel 73 through an insulating film 74. A p-type channel stop region 76 is formed between the adjacent photoelectric converting units 71.
The channel stop region 76 serves to carry out an electrical isolation of the photoelectric converting unit 71 and the vertical transfer channel 73. The insulating film 74 is an ONO film obtained by laminating, on a surface of the semiconductor substrate 81, a silicon oxide film formed by a thermal oxidation, a silicon nitride film formed by CVD, for example, and a silicon oxide film obtained by the thermal oxidation of a surface of the silicon nitride film, for example, in this order from below. The vertical transfer electrode 75 includes a first layer vertical transfer electrode and a second layer vertical transfer electrode which are formed by polysilicon, for example. An insulating silicon oxide film 77 obtained by the thermal oxidation of polysilicon is formed on the vertical transfer electrode 75, for example. The vertical CCD portion 64 is constituted to include the vertical transfer channel 73, and the insulating film 74 and the vertical transfer electrode 75 which are formed thereon.
A light shielding film 79 is formed of tungsten, for example, through the insulating silicon oxide film 77 above the vertical transfer electrode 75. The light shielding film 79 has an opening portion 79a formed above the photoelectric converting unit 71. A silicon nitride film 78 is formed on the light shielding film 79.
A signal charge generated in the photoelectric converting unit 71 corresponding to an amount of an incident light is transferred into the vertical transfer channel 73 through a driving signal (a transfer voltage) read from the reading gate 72 to the vertical transfer channel 73 and applied to the vertical transfer electrode 75. The light shielding film 79 has the opening portion 79a above each photoelectric converting unit 71 as described above and prevents a light incident on the pixel array portion 61 from being incident on a region other than the photoelectric converting unit 71.
A flattened layer 83a formed of silicon oxide is provided above the light shielding film 79, for example, and a color filter layer 84 for three primary colors of red (R), green (G) and blue (B) is formed on the flat surface, for example. In order to flatten a portion provided thereon, furthermore, a flattened layer 83b is formed. A microlens 85 formed of a photoresist for a microlens is provided on the flattened layer 83b having a flattened surface, for example. In the microlens 85, a very small hemispherical convex lens is arranged above each photoelectric converting unit 71, for example. The microlens 85 collects the incident light into the photoelectric converting unit 71. The light to be collected by the microlens 85 is incident on the photoelectric converting unit 71 through the color filter layer 84. A reactive space 85a is formed between the adjacent microlenses 85.
In order to manufacture the microlens 85, some methods have been known. There has been a method of patterning a photoresist for a microlens having both a photosensitivity to an i-line and a thermosetting property to take a planar shape of the lens and then carrying out a heat treatment to cause a surface to be a spherical surface, thereby obtaining the shape of the lens.
Moreover, there has also been proposed a technique for applying a photoresist for a microlens onto a transparent lens material member and carrying out patterning to form an original shape of the lens, and then transferring a shape onto the lens material member through dry etching (for example, see JP-A-10-148704).
On the other hand, a reduction in the reactive space 85a of the microlens 85 has been required with the microfabrication of a pixel in an image pick-up device. However, a photoresist for a microlens which is currently put on the market is used exclusively for the i-line and has a low resolution. For this reason, it is hard to sufficiently reduce the reactive space 85a. 
With the microfabrication of a pixel, moreover, the use of a KrF exposing device in a wafer process has been a mainstream and the opening portion 79a of the light shielding film 79 is formed by using the KrF exposing device, for example. For this reason, in the case in which a special photoresist for the i-line and the i-line exposing device are used in order to form the microlens 85, it is hard to obtain high precision in an alignment between an opening pattern of the opening portion 79a of the light shielding film 79 and the microlens 85 by the influence of a peculiar distortion to the device and a magnification error.